Available online at www.sciencedirect.com ScienceDirect Physics Procedia 66 (2015 ) 489 493 C 23rd Conference on Application of Accelerators in Research and Industry, CAARI 2014 Future Upgrades for the PHENIX Experiment at RHIC: From PHENIX to sphenix And Beyond Achim Franz for the PHENIX collaboration1 Brookhaven National Laboratory, Physics Dept., Upton NY 11973-5000 Abstract A series of major upgrades are being planned for the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) to enable a comprehensive measurement of jets and quarkonia in relativistic heavy ion collisions in order to study the phase transition of normal nuclear matter to the Quark Gluon Plasma near its critical temperature. These upgrades will include a number of major new detector systems. The former BaBar solenoid magnet will be incorporated in the first stage, sphenix. Further upgrades will include two new large calorimeters. In the following these upgrades and their requirements are being presented. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license 2014 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014. Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014 Keywords RHIC, heavy ion, QGP, jets, calorimetry, R&D 1. Introduction Over the past 15 years the Relativistic Heavy Ion Collider (RHIC) (Harrison, 2003) at Brookhaven National Laboratory (BNL) on Long Island, NY has produced a wide variety of insights into the physics of the Quark Gluon Plasma (QGP) and the spin configuration of the proton (Rak, 2013) (Aschenauer, 2013). RHIC, being the last high energy particle, and the only polarized proton collider operating in the US, has proven to be the most versatile in the field of heavy ion physics as its dual superconducting rings permit a variety of nuclei to be brought into collision, * Corresponding author. Tel.+1-631-344-4750; fax: +1-631-344-3253. E-mail address: afranz@bnl.gov 1875-3892 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014 doi:10.1016/j.phpro.2015.05.063
490 Achim Franz / Physics Procedia 66 ( 2015 ) 489 493 starting from spin polarized protons up to U with mixed beams of d-au, Cu-Au and - new in 2014-3 He-Au. In the coming years p-a collisions will be available too. Beam energies range from 5.5GeV to 100GeV/A for ions and up to 255GeV for polarized proton beams. We learned that the resulting plasma is not a weakly interacting gas of quarks and gluons but rather a strongly coupled plasma which flows and expands more like a liquid at a measured 3 Trillion degrees. Studying the properties of the short lived plasma (~ 10-24 sec), requires new and innovative tools and one of the most commonly used are jets from parton collisions. Jets are well known from lepton and hadron colliders; after the collision the parton pair flies off in opposite directions, and materializes as a well-defined and confined cone of particles with equal total energy. In heavy ion collisions this parton scattering happens inside the hot and dense plasma which leads to energy and momentum losses for the jets. This energy and momentum loss is a measure of the density, viscosity and coupling of the QGP (Coleman-Smith, 2012). Finding jets in heavy ion collisions with a charged particle density of ~ 600 and a fluctuating energy background of about 500GeV per unit pseudorapidity (η) for s NN = 200GeV Au + Au collisions, requires a hermetic detector coverage and something none of the RHIC detectors currently have, namely complete electromagnetic (EMCal) and hadronic (HCal) calorimetry. With the PHENIX detector close to its limitations on these physics topics, planning has begun for a new specialized detector, while making use of as much of the current PHENIX infrastructure as possible. 2. sphenix physics requirements A detector to tackle these physics questions would need: - large uniform acceptance - HADCal and EMCal with good energy resolution and segmentation - trigger capabilities for jets, electrons and photons - good track reconstruction efficiency and purity - good momentum resolution - capability to detect displaced vertices - high rate DAQ system Working around the available 1.5T BaBar solenoid magnet, which is 384cm long with a 140cm inner radius, was signed over to BNL from SLAC and should arrive in late summer 2014, we designed the detector concept seen in Fig.1 (Adare, 2012) which fulfills the above requirements. 3. sphenix design details Detailed jet detector simulations have shown (Hanks, 2012) that in 200GeV 2 Au 2 + Au collisions at RHIC real jets dominate over fake ones for E jet > 20GeV for a jet cone radius R = Δφ + Δη of 0.2, and E jet > 40GeV for a jet cone radius of 0.4. Unfolding these jets from a fluctuation background of up to 500GeV per unit η sets the requirements for the HCal at ~ 100%/ E with a segmentation of dη x dφ = 0.1 x 0.1 in two (maybe three) radial segments (see Fig.2). Studies of Hadron-Jet and γ-jet correlations yield similar detector requirements. One of the QGP signatures is the disappearance of the quarkonia states specially the three Upsilon states which are expected to melt at different ranges of the QGP temperature. To separate the three states, large acceptance, good tracking and momentum resolution together with a finely segmented EMCal are necessary. We estimate that the requirements are δp T /p T ~ 0.2% for the momentum resolution and ~15%/ E for the EMCal with a segmentation of dη x dφ = 0.025 x 0.025. Having such a small segmentation at a radius of only 90cm requires a high density material, and an obvious choice is tungsten with a 7mm radiation length and a Molière radius of 2.4cm. Tracking will be provided by several layers of silicon detectors from the enlarged and reconfigured vertex tracker currently in use at PHENIX.
Achim Franz / Physics Procedia 66 (2015 ) 489 493 491 Fig. 1. Baseline sphenix detector inside the PHENIX interaction region. Visible are, from the inside out, the Silicon tracker (green), EMCal (blue), inner HCal (brown), solenoidal magnet (grey), and the outer HCal (red) see also Fig.2. Fig. 2. Cross section of the baseline sphenix the radial positions are indicated on the right [cm]. To acquire the necessary statistics we need a DAQ system capable of close to 10kHz event rate; currently PHENIX takes data at up to 8kHz. Over a typical 20 week / year running period sphenix could record over 50
492 Achim Franz / Physics Procedia 66 ( 2015 ) 489 493 billion and sample over 200 billion Au + Au collisions at s NN = 200 GeV or 10 7 dijet events for E jet > 20GeV and a correspondingly large γ-jet sample. The EMCal will be based on the current UCLA Tungsten - Scintillator Fiber (WSci) calorimeter (Tsai, 2012) which has shown to have the required resolution and can easily be segmented to the required tower size. The scintillating fibers are embedded in a W-epoxy mixture, bundled to the required tower size and read out by multiple silicon photomultipliers (SiPM) per tower. The complete EMCal, including readout, will be at radius 90-113.5cm equaling 18 radiation length X 0 or 1 interaction length λ 0, followed by the first segment of the HCal at radius 113.5-140cm with a depth of 1λ 0. The outer HCal has a thickness of 4λ 0 outside the magnet and is planned to be read out on either side, adding additional segmentation. Using the external HCal as the flux return for the magnet (see Fig.1) we settled on the following unique and novel designs. It s made of long tilted metal plates running along the beam direction with gaps for scintillator tiles, read out by embedded wavelength shifting fibers and SiPMs (see Fig.3). 4. Prototypes and Test beam Fig. 3. sphenix calorimeter layout. A prototype of the HCal and an alternate EMCal design was tested at the FNAL MT6 Test beam facility in early 2014 with beams of electrons and hadrons ranging from 1 to 120GeV/c, with the highest energy coming from the primary beam of the Main Injector and consisting of mainly protons. A beam line Cherenkov counter enabled the identification of electrons and pions between 1 and 32 GeV/c. The HCal prototype, seen in Fig.4 consisted of alternating steel plates with scintillator tiles. The steel plates varied from 2cm in the front to 5cm in the back with 8.5mm gaps for the 7mm thick scintillator tiles. These tiles were coated with UV reflective coating and had a U-shaped groove in which a wavelength shifting fiber was embedded. The fibers were bundled to form a 4x4 array of towers in the front and back, read out by a single SiPM, each attached to a small light mixing cavity. The entire prototype measured 1.2 x 1.7 x 1.3 m 3 and weighed approximately 4 tons. The EMCal prototype we tested was of a tilted tungsten plate design read out by alternating ribbons of scintillator fibers, both 1mm thick, and SiPMs coupled to the fibers by an air cavity. Even though the analysis is still ongoing, preliminary results show a good agreement between these test beam data and simulations.
Achim Franz / Physics Procedia 66 (2015 ) 489 493 493 Fig. 4. Left: Steel plates for the HCal prototype calorimeter. Right: Layout of the prototype HCal and EMCal along the beam line in the test beam at FNAL References Harrison M., Ludlam T. and Ozaki S., 2003, The Relativistic Heavy Ion Collider Project: RHIC and its Detectors, NIM 499, Issues 2-3, Pages 235-880 Rak J., Tannenbaum M.J., 2013, High-pT Physics in the Heavy Ion Era, Cambridge Monographs on Particle Physics, Nuclear Physics and Cosmology, isbn: 9780521190299 Aschenauer E.C. et al., 2013, The RHIC Spin Program: Achievements and Future Opportunities, arxiv:1304.0079 [nucl-ex] 2013 Coleman-Smith C.E. and Müller B., 2012, Results of a systematic study of dijet suppression measured at the BNL Relativistic Heavy Ion Collider, Phys. Rev. C 86, 054901 Published 5 November 2012 Adare A. et al., 2012, sphenix: An Upgrade Concept from the PHENIX Collaboration, arxiv:1207.6378v2 [nucl-ex] Hanks A. et al., 2012, Method for separating jets and the underlying event in heavy ion collisions at the BNL Relativistic Heavy Ion Collider, Phys. Rev. C86 (2012) 024908 Tsai O.D., Dunkelberger L.E., Gagliardi C.A., Heppelmann S., Huang H.Z. et al., 2012, Results of on a new construction technique for W/ScFi Calorimeters. J.Phys.Conf.Ser., 404:012023, 2012. doi:10.1088/1742-6596/404/1/012023. 3.2.1